Principle Of Internet

Port Numbers
To identify a host machine, an IP address or a domain name is needed. To identify a particular server on a host, a port number is used. A port is like a logical connection to a machine. Port numbers can take values from 1 to 65,535. It has no correspondence with the physical connections, of which there might be just one. Each type of service has, by convention, a standard port number. Thus 80 usually means Web serving and 21 means file transfer. If the default port number is used, it can be omitted in the URL (see below). For each port supplying a service there is a server program waiting for any requests. Thus a web server program listens on port 80 for any incoming requests. All these server programs run together in parallel on the host machine.

When a packet of information is received by a host, the port number is examined and the packet sent to the program responsible for that port. Thus the different types of request are distinguished and dispatched to the relevant program.

The following table lists the common services, together with their normal port numbers. These conventional port numbers are sometimes not used for a variety of reasons. One example is when a host provides (say) multiple web servers, so only one can be on port 80. Another reason is where the server program has not been assigned the privilege to use port 80.

Sockets
A socket is the software mechanism for one program to connect to another. A pair of programs opens a socket connection between themselves. This then acts like a telephone connection - they can converse in both directions for as long as the connection is open. (In fact, data can flow in both directions at the same time.) More than one socket can use any particular port. The network software ensures that data is routed to or from the correct socket.

When a server (on a particular port number) gets an initial request, it often spawns a separate thread to deal with the client. This is because different clients may well run different speeds. Having one thread per client means that the different speeds can be accommodated. The new thread creates a (software) socket to use as the connection to the client. Thus one port may be associated with many sockets.

Streams
Accessing information across the Internet is accomplished using streams. A stream is a serial collection of data, such as can be sent to a printer, a display or a serial file. Similarly a stream is like data input from a keyboard or input from a serial file. Thus reading or writing to another program across a network is just like reading or writing to a serial file.

URL
A URL (Uniform Resource Locator) is:

a unique identifier for any resource on the Internet
typed into a Web browser
used as a hyperlink within a HTML document
quoted as a reference to a source

A URL has the structure:

protocol: //hostname[:port]/[pathname]/filename#section

The host name is the name of the server that provides the service. This can either be a domain name or an IP address.

The port number is only needed when the server does not use the default port number. For example, 80 is the default port number for HTTP.